1. Field of the Invention
The present invention relates to an optical source generator for wavelength-division-multiplexing (WDM) optical communication systems.
2. Description of the Related Art
Recent research has actively focused on increasing transmission capacity in an optical field through the use of a multi-channel optical source that is subject to WDM. In WDM, each optical signal to be transmitted is allocated a respective wavelength so that multiple signals can flow simultaneously on a single channel. Currently, a semiconductor laser is generally used as the optical source of the transmitter in a WDM optical communication system. However, the semiconductor laser optical source needs precise wavelength control to operate at a wavelength recommended by the International Telecommunication Union (ITU) and to allow the output wavelength to be subject to temperature control. If a multi-channel optical source is required, the number of wavelengths to be controlled increases, which complicates the controlling operation. In addition, if a multiplexed multi-channel optical source is needed, so is a separate multiplexer.
To solve these problems, a multi-wavelength laser optical source generator has recently been developed which employs a plurality of fiber-Bragg gratings (FBGs) and erbium-doped fiber amplifiers (EDFAs).
FIG. 1 shows a conventional multi-wavelength laser optical source generator. It is designed with fiber-Bragg gratings 4A, 4B, and 4C which are each configured to transmit a wavelength that meets ITU recommendations. The optical source generator is further configured with erbium-doped optical fiber amplifiers 3A, 3B, and 3C that are each interposed between the fiber-Bragg gratings 4A, 4B, and 4C or vice versa. A single pump laser is used for optical fiber amplification. As shown in FIG. 1, the optical source generator also includes a pump laser 1, a wavelength-division multiplexer/demultiplexer 2, an attenuator 6, and a polarization controller 8.
As seen from FIG. 1, spontaneously emitted lights generated from the optical fiber amplifiers 3A, 3B, and 3C are primary-reflected by means of the fiber-Bragg gratings 4A, 4B, and 4C, and then secondary-reflected by a mirror 5 to the left of the fiber-Bragg gratings 4A, 4B, and 4C. The secondary-reflected lights are then tertiary-reflected by the fiber-Bragg gratings 4A, 4B, and 4C. Accordingly, the spontaneously emitted lights may be subject to numerously repetitive lasing. Consequent gains by means of optical fiber amplifiers 3A, 3B, and 3C allow subsequent use of the lights as laser optical sources. Each of the optical fiber amplifiers 3A, 3B, and 3C is situated in the corresponding wavelength-compatible resonant cavity among those resonant cavities that are defined between the fiber-Bragg gratings 4A, 4B, and 4C and the mirror 5. For example, in FIG. 1, an optical source having a wavelength of λ1 is subjected to lasing between the first fiber-Bragg grating 4A and the mirror 5 and makes use of the first erbium-doped fiber amplifier 3A as an amplifier medium. Similarly, an optical source having a wavelength of λ2 is subjected to lasing between the second fiber-Bragg grating 4B and the mirror 5 and makes use of the first erbium-doped optical fiber amplifier 3A and the second erbium-doped optical fiber amplifier 3B. The respective single wavelength optical sources generated in this way make use of the same optical fiber amplifiers and mirror so that they are multiplexed and reflected. Therefore, these multiplexed optical sources are extracted by a coupler 7 disposed between the mirror 5 and the fiber-Bragg gratings 4A, 4B, and 4C.
However, in the multi-wavelength optical source generator according to the prior art as mentioned above, a single amplifier can be shared by a plurality of optical sources. Consequently, operation of the amplifier within a saturation region to achieve high output for one channel may lead to a change in gain of another channel, which can cause each optical source power to fluctuate unstably. Moreover, as a plurality of generated and multiplexed optical sources are extracted through the coupler, generally only the use of multiplexed optical sources is feasible. It is difficult to apply conventional multi-wavelength optical source generators to optical communication systems that employ individual optical sources.